UNIX Introduction
What is UNIX?
UNIX® License PlateUNIX is an operating system which was
first developed in the 1960s, and has been under constant development ever
since. By operating system, we mean the suite of programs which make the
computer work. It is a stable, multi-user, multi-tasking system for servers,
desktops and laptops.
UNIX systems also have a graphical user interface (GUI)
similar to Microsoft Windows which provides an easy to use environment.
However, knowledge of UNIX is required for operations which aren't covered by a
graphical program, or for when there is no windows interface available, for
example, in a telnet session.
Types of UNIX
The Linux PenguinThere are many different versions of UNIX,
although they share common similarities. The most popular varieties of UNIX are
Sun Solaris, GNU/Linux, and MacOS X.
Here in the School, we use Solaris on our servers and
workstations, and Fedora Linux on the servers and desktop PCs.
The UNIX operating
system
The UNIX operating system is made up of three parts; the
kernel, the shell and the programs.
The kernel
The kernel of UNIX is the hub of the operating system: it
allocates time and memory to programs and handles the filestore and
communications in response to system calls.
As an illustration of the way that the shell and the kernel
work together, suppose a user types rm myfile (which has the effect of removing
the file myfile). The shell searches the filestore for the file containing the
program rm, and then requests the kernel, through system calls, to execute the
program rm on myfile. When the process rm myfile has finished running, the
shell then returns the UNIX prompt % to the user, indicating that it is waiting
for further commands.
The shell
The shell acts as an interface between the user and the
kernel. When a user logs in, the login program checks the username and
password, and then starts another program called the shell. The shell is a
command line interpreter (CLI). It interprets the commands the user types in
and arranges for them to be carried out. The commands are themselves programs: when
they terminate, the shell gives the user another prompt (% on our systems).
The adept user can customise his/her own shell, and users
can use different shells on the same machine. Staff and students in the school
have the tcsh shell by default.
The tcsh shell has certain features to help the user
inputting commands.
Filename Completion - By typing part of the name of a
command, filename or directory and pressing the [Tab] key, the tcsh shell will
complete the rest of the name automatically. If the shell finds more than one
name beginning with those letters you have typed, it will beep, prompting you
to type a few more letters before pressing the tab key again.
History - The shell keeps a list of the commands you have
typed in. If you need to repeat a command, use the cursor keys to scroll up and
down the list or type history for a list of previous commands.
Files and processes
Everything in UNIX is either a file or a process.
A process is an executing program identified by a unique PID
(process identifier).
A file is a collection of data. They are created by users
using text editors, running compilers etc.
Examples of files:
·
a document (report, essay etc.)
·
the text of a program written in some high-level
programming language
·
instructions comprehensible directly to the
machine and incomprehensible to a casual user, for example, a collection of
binary digits (an executable or binary file);
·
a directory, containing information about its
contents, which may be a mixture of other directories (subdirectories) and
ordinary files.
The Directory
Structure
All the files are
grouped together in the directory structure. The file-system is arranged in a hierarchical
structure, like an inverted tree. The top of the hierarchy is traditionally
called root (written as a slash / )
In the diagram above,
we see that the home directory of the undergraduate student "ee51vn" contains two sub-directories (docs and pics)
and a file called report.doc.
The full path to the
file report.doc is "/home/its/ug1/ee51vn/report.doc"
Unix File Structure
In the diagram above, we see that the home directory of the
undergraduate student "ee51vn" contains two sub-directories (docs and
pics) and a file called report.doc.
The full path to the file report.doc is
"/home/its/ug1/ee51vn/report.doc"
Starting an UNIX terminal
To open an UNIX terminal window, click on the
"Terminal" icon from Applications/Accessories menus.
